Why all the paranoid people on head gaskets??? Geez every time someone has a heating or coolant problem a head gasket seems to come up as the suspect. Head gasket failure is not all that common. sure it happens but no near as often as it seems that it is brought up here. If this would jhave been a head gasket with compression blowing the coolant out there would also have been steam or coolant or residue from the exhaust as some point. If it can blow pressure into the cooling system at some point it will also allow coolant the other way.
Rereading the post I am also in fafor of the poor seal on the stat housing. I have seen many times where the edge of tha stat got caught between the stat housing and the w/pump housing. Just enough to cause a leak under pressure.